A Study For Modeling And Functional Simulating Of Human Crystalline Lens

The research of virtual organs is a very important domain of modern biomedical engineering. As human eye is one of the most important human organs, the research of virtual eye is very meaningful and valuable. The construction of virtual eye system requires the most advanced technologies in many fields and it is very important to theoretical researches and clinical applications of ophthalmologic physiology and pathology. Human crystalline lens is one of the most important components of a human eye for it is the key part of accommodation mechanism of a human eye. Furthermore, there are direct relationships between human crystalline lens and many eye diseases such as presbyopia and catalyst. Therefore, as an important part of virtual eye research, to model and simulate human crystalline lens can be very useful both to the study of human eye's functions and to the research of treatment to concerned eye diseases.This paper studied the geometric modeling, physical modeling and functional simulation of the human crystalline lens system in order to construct the 'crystalline lens component' of virtual human eye. Another purpose is to provide reference for the study of other components of virtual eye on the technology approach.The main contents and contributions of this dissertation are as follows:1. Parametric modeling method was introduced to geometric modeling of human crystalline lens based on multi-source data fusion. 5 key parameters were extracted to characterize lens geometry, including lens diameter, lens thickness, nucleus diameter, nucleus thickness and nucleus shift. 2D profile of human crystalline lens was described on the basis of these 5 parameters. By rotating the constructed 2D profile, 3D solid model of human crystalline lens then was constructed to display different lens.2. With the construction of geometric model, physical model of 'lens---zonules---ciliary body' system was then generated to simulate the accommodation mechanism of human crystalline lens by finite element method. Theoretical finite element analysis was firstly applied to the physical model to study the properties of its solution. The analysis led to the following conclusion: the solution of current physical model changes linearly both with the force load and with the pressure load; and the influence of the force load and the pressure load to the solution can accumulate linearly. By applying theoretical results, a simplification computing method of solving the model was proposed to reduce the computation cost.3. By applying 29 years old and 45 years old physical models, simulation was conducted to study two hypothesis of the accommodation mechanism: Helmholtz's hypothesis and Schachar's hypothesis. The study of Helmholtz's simulation showed that Helmholtz's hypothesis accords with two facts of accommodation which are popularly approved. And these basic rules of accommodation were robust. Only the numerical range was influenced by model's parameters. This indicated that Helmholtz's hypothesis was perfectly supported by the simulation of this paper. The study of Schachar's hypothesis also came to the accordance with current two facts of accommodation. However, basic rules of accommodation achieved by the simulation of Schachar's hypothesis differed when the parameters changed. Therefore, further studies both on clinical research and simulation research are expected. Comparison of different age model showed that, as a human turns old, the variation happening to his crystalline lens both in the shape and in the material properties leads to the decreasing of human eye's accommodation ability, as current presbyopia theory describes. The influence of eye pressure on the lens was studied by applying physical model of human crystalline lens. The simulation results showed that, the pressure has a great influence on the lens shape as well as its optical power. And as the pressure increased, the lens got thicker and its diameter increased; the curvature radius of anterior and posterior surfaces increased and the optical power of the lens increased. Furthermore the simulation of Helmholtz' hypothesis with pressure indicated that the influence of pressure was independent to the influence of the force load and the basic rule was identical with the case of zero pressure.4. A nonlinear, large deformation model of the anterior capsule was constructed to study the relationship between CCC diameter and the maximum extend of the anterior capsule. Simulation results showed that, the larger the CCC diameter was, the greater the capsule could be extended, which was accord with the results of experimental studies on body lens capsule. Furthermore, simulation indicated that, with the same CCC diameter, the capsule extended greater as the contact area of the external force got larger. This result remains undiscovered in experimental studies.5. A computer software system was developed to display the human crystalline lens component and to simulate the accommodation. User can modify the lens shape by inputting different parameters and can experience the accommodation process interactively.The main creativity in this paper is the following:1. Introduced parametric modeling into the geometric modeling of human crystalline lens and realized the parameter-based construction of different lens. Base on the parametric modeling method, various human crystalline lenses were generated and displayed on computer.2. A physical model was constructed to simulate the lens accommodation and got useful theoretical results of the constructed model by finite element analysis. Theoretical analysis proved that the solution of the current model varied linearly with the displacement load and pressure load. And on the basis of theoretical results, a fast computation method was proposed to simplify the model solving.3. By applying physical model, Helmholtz's hypothesis was perfectly supported by simulation results. And physical model was proved to be robust and reasonable. Schachar's hypothesis was not supported as well as Helmholtz's hypothesis and simulation results depended on the elastic property of the zonules.4. Eye pressure was found to influence the lens shape and its optical power remarkably, which was never mentioned before. And the influence of the pressure load and the force load were independent to each other. This led to the result that the pressure didn't influence the basic rules of the simulation of Helmholtz's hypothesis.5. Nonlinear finite element model was constructed to study the extension of CCC capsule for the first time and simulation got the same results as in experimental studies. And with the same CCC diameter, the capsule extended greater as the contact area of the external force got larger.